Nitrile rubbers (“NBR”) are rubbers which are co- or terpolymers of at least one α,β-unsaturated nitrile, of at least one conjugated diene and, if appropriate, of one or more other copolymerizable monomers. Hydrogenated nitrile rubbers (“HNBR”) are corresponding nitrile rubbers in which the C═C double bonds of the diene units incorporated into the polymer have been completely or partially selectively hydrogenated.
Both NBR and HNBR have for many years occupied a secure position in the sector of speciality elastomers. They have an excellent property profile in the form of excellent oil resistance, good heat resistance and outstanding resistance to ozone and chemicals, and this latter resistance is even higher for HNBR than for NBR. Furthermore, they have very good mechanical properties, and also good performance characteristics. They are therefore widely used in a very wide variety of application sectors, and by way of example are used for producing gaskets, hoses, drive belts and damping elements in the automobile sector, and also for stators, borehole seals and valve seals in the oil-production sector, and also for numerous components in the electrical industry, and also in mechanical engineering and shipbuilding. There is a wide variety of commercially available types, and these feature different monomers, molecular weights, polydispersities, and also mechanical and physical properties, as a function of application sector. In particular, there is increasing demand not only for the standard types but also for speciality types comprising specific termonomers or particular functionalization.
Industrial production of nitrile rubbers proceeds almost exclusively via what is known as emulsion polymerization, which is carried out in the presence of relatively large amounts of emulsifiers. After the polymerization process, the resultant NBR latex, which is a suspension of solid polymer particles in water, stabilized by the emulsifier present, is coagulated in a first step which uses salts or acids, and the solid NBR is isolated. If the intention is to proceed to hydrogenation of the NBR to give HNBR, the said hydrogenation likewise uses known prior-art methods, for example, with use of homogeneous or else heterogeneous hydrogenation catalysts based on rhodium, ruthenium or titanium or alternatively platinum, iridium, palladium, rhenium, osmium, cobalt or copper, either in the form of metal or else in the form of metal compounds. In industry, the said hydrogenation is often carried out in a homogenous phase, i.e. in an organic solvent.
The use of HNBR for producing V-belts, toothed belts and conveyor belts is known. These items are composed of a combination of a substrate material made of fibres (e.g. polyamide textile, polyester textile, glass cord, aromatic polyamide cord, etc.) and of a plurality of rubber layers. There are other rubber items having a similar structure, examples being membranes, hoses, containers, balloons and tyres. In all instances it is important to design the bond between the surface of the substrate material and the rubber in such a way that it is not the point of weakness within the composite system. Most substrate materials used nowadays, including those used in tyre production, are therefore first treated with latices, and there is therefore increasing demand for these. A very particularly important factor here is good adhesion of the rubber to the surface of the substrate material. These latices are suspensions of the appropriate solid rubber particles in water. They are often used in binder compositions which also comprise resins/hardeners and which are known in the literature as “RFL dips” (“resorcinol-formaldehyde-latex dips”) (see, for example, EP-A-0 381 457).
U.S. Pat. No. 4,452,950 describes a process for hydrogenating the carbon double bonds in a polymer which is present in the form of a latex. The hydrogenation process uses an oxidant, a reducing agent in the form of hydrazine or hydrazine hydrate and a metal ion inhibitor. By way of example, it is possible to use NBR latices which are obtained via aqueous emulsion polymerization, and it is preferable to use these without prior coagulation and without use of organic solvent (column 3, lines 8-11). These latices are therefore suspensions of the emulsifier-stabilized NBR particles in water. The hydrogenation process therefore correspondingly gives an HNBR latex. The examples show that the achievable degree of hydrogenation is at most 82%, i.e. at least 18% of the double bonds are retained. The reason for the relatively low degrees of hydrogenation is that gel content rises markedly at higher conversions. Gel particles within the latex weaken the mechanical properties of rubber products produced therefrom. Since the NBR emulsion polymerization process is carried out in the presence of emulsifiers, the resultant HNBR latex necessarily also comprises emulsifiers, with the attendant disadvantages described at a later stage below. HNBR latices produced in this way do not therefore comply with all of the requirements.
Production of latices, i.e. suspensions of solid rubber particles in an aqueous phase, also uses shear processes. Here, an organic solution of the polymer in the form of liquid organic phase is brought into contact with an aqueous phase, using a high shear rate. Emulsifiers or emulsifier mixtures with other auxiliaries are present here either in the aqueous phase only or in both phases, in order to improve the emulsifying effect. Stripping, depressurization or other distillative methods are used to remove the solvent from emulsions obtained by the said route, and the suspensions thus produced are therefore initially of relatively low concentration (another term sometimes used in the literature being “thin latex”). These suspensions are converted to the desired final concentration by way of example via further distillation, centrifuging or creaming processes. However, emulsification processes of this type are attended by high shear rates since this is the only way of obtaining fine-particle, stable emulsions (and corresponding suspensions after concentration), and their use in industry is very resource-intensive in terms of apparatus and energy. Relatively large amounts of emulsifiers are moreover required. This is disadvantageous because the latter remain in the form of a water-soluble constituent in the compounded rubber materials, but are not concomitantly vulcanized, and the result can therefore be impairment of the mechanical properties of the finished rubber component. They also lead to undesired caking of contaminants within the moulds used to produce the rubber components. Another particular disadvantage is that residues of emulsifiers can, by virtue of their surfactant properties, have an adverse effect on the adhesion of HNBR to substrate materials and in particular reinforcement fibres.
EP-A-0 240 697 describes analogous production of a highly saturated nitrile rubber latex. The highly saturated nitrile rubbers used comprise copolymers having repeat units of an α,β-unsaturated nitrile and of a conjugated diene, and corresponding terpolymers with additional repeat units of one or more copolymerizable termonomers are also used. A solution of the highly saturated nitrile rubber, the iodine number of which is not more than 120, in an inert solvent which is not water-miscible is added to emulsifier-containing water. Mechanical emulsification, i.e. emulsification achieved by introducing energy, is used to form an oil-in-water emulsion, which is therefore a liquid-liquid emulsion. The solvent is then removed from this via conventional distillation methods (stripping with steam or “steam stripping”), and the remaining aqueous phase is concentrated. The rubber molecules are then present with stabilization via emulsifier molecules in water, this being a solid-liquid system. The aqueous phase can comprise further excess amounts of emulsifier, alongside the amount of emulsifier adsorbed on the surface of the rubber particles and necessary for stabilizing the rubber particles. Centrifuging is then used to remove water from the latex, and the concentration of the said excess emulsifier present in this water is the same as in the water that remains within the latex. The centrifuging process therefore removes only that portion of the emulsifier which is not adsorbed on the surface of the rubber and is not necessary for stabilizing the rubber particles within the water. There is therefore a limit on the maximum amount of emulsifier that the centrifuging process can actually remove. If a further attempt were to be made to withdraw still further amounts of emulsifier from the latex via centrifuging or via other methods, the result would be coagulation of the rubber, therefore giving particles with average diameters far greater than 1 micrometer. This classical route given in EP-A-0 240 697 for producing latices does not permit production of fine-particle rubbers which have only very low emulsifier content but which nevertheless form stable suspensions. The description of EP-A-0 240 697 says that in order to obtain a stable latex it is necessary to use from 1 to 20 parts by weight of the emulsifier, based on 100 parts by weight of HNBR, and also to set a pH of from 8 to 13. In Example 2, emulsifier concentration is 5%, based on the rubber. The solution of the saturated nitrile rubber used for latex production can either be the solution obtained at the end of the hydrogenation process after optional further dilution, or can be a solution obtained via dissolution of solid, previously isolated HNBR in a solvent. The examples of EP-A-0 240 697 use a solvent mixture made of toluene and dichloroethane or of cyclohexane and methyl ethyl ketone. The description mentions in principle, as solvents, aromatic solvents, such as benzene, toluene and xylene, halogenated hydrocarbons, such as dichloroethane and chloroform, or ketones, such as methyl ethyl ketone and acetone, and also tetrahydrofuran (THF). According to our own investigations, however, it is not possible to use either acetone or THF for the process described in EP-A-0 240 697, since both solvents are soluble in any ratio with water, and therefore cannot per se form an emulsion, and the entire principle of the process of EP-A-0 240 697 is therefore inapplicable. The emulsifiers used comprise anionic emulsifiers, if appropriate in combination with nonionic emulsifiers. The size (i.e. the diameter) of the suspended rubber particles stabilized via appropriate amounts of emulsifier within the latex is determined by the emulsification conditions used, i.e. the amount of water and of emulsifier, and also the amount of energy introduced (e.g. in the form of agitation rate), and is stated as from 0.05 to 5 μm. It is significant that these are emulsifier-stabilized polymer particles in suspension. The HNBR contains no, or only very little, gel. The experimental description in the examples includes emulsifier-stabilized latices with about 45% solids content (rubber), with pH in the range from 9 to 9.5 and with an average particle diameter of from 0.32 to 0.61 μm, determined by an electron microscope. Another disadvantage of the said process is that particle size distribution is necessarily broad when mechanical emulsification processes are used. Furthermore, exchange of substances from the particles is inhibited by emulsifiers at the phase boundary, and this makes it more difficult to use conventional processes of concentration by evaporation or by distillation to remove solvent retained within the particles. Because of the emulsifier content, foaming is likely to occur during the concentration process, and this adds to the difficulty of achieving solvent removal under practical conditions, in particular under industrial conditions. Finally, the centrifuging step is also problematic. EP-A-0 240 697 states a requirement for 15 minutes at 3000 revolutions per minute to remove excessive emulsifier and to concentrate the latex, and this is a considerable obstacle to industrial-scale production. The fact that a centrifuge can be used to concentrate the latex necessarily also implies restricted storage stability in respect of sedimentation. As described above, it will not be possible to remove the emulsifier completely. However, emulsifier residues are disadvantageous, as described above.
The process described in EP-A-0 704 459 is in principle the same as that described in EP-A-0 240 697 for producing aqueous emulsifier-stabilized HNBR latices, the term used in the said document being “phase reversal of emulsion”. Again according to EP-A-0 285 094 the production of HNBR co- or terpolymer latices is achieved by mixing an organic solution of the HNBR with an emulsifier-containing aqueous phase, thus obtaining a liquid-liquid oil-in-water emulsion, and then removing the solvent, thus obtaining an emulsifier-containing HNBR suspension in water. To obtain a stable suspension, the emulsification process must be carried out with a high level of energy introduction by means of vigorous agitation. The method of further work-up is identical with that in EP-A-0 240 697. The latices obtained have solids concentrations of about 45% by weight and pH of from 9 to 9.5. Neither EP-A-0 704 459 nor EP-A-0 285 094 provides any teaching or information about the possibility of producing fine-particle HNBR latices which have adequate stability and do not agglomerate, without use of relatively large amounts of emulsifier.
EP-A-0 252 264 also discloses the production of HNBR latices for coating substrate materials. Here, the HNBR polymer is first dissolved in an organic solvent or a solvent mixture, which either has low water-solubility and forms an azeotrope with water, with more than 50% content of the solvent, or has a boiling point below 95° C. Organic solvents mentioned are 3-chlorotoluene, diisobutyl ketone, methyl isobutyl ketone and methylisopropyl ketone. The resultant organic phase is emulsified in water by techniques known per se using anionic, cationic or nonionic emulsifiers or methylcellulose. The organic solvent is then removed via known methods, such as distillation. By way of example, this route can be used to obtain latices with up to 19% solids content and with an average particle size of 480 μm (Example 2). However, this type of particle size is much too large to justify the term “latex” in the narrower sense. The latex is therefore unsuitable as component for a dip for industrial coating of substrate materials. The said process also has the same emulsifier-content problems as the process of EP-A-0 240 697.
EP-A-0 863 173 discloses a process for producing stable, fine-particle emulsifier-stabilized polymer suspensions with polymer particle sizes of from 0.1 to 50.0 μm, particularly preferably from 0.1 to 2 μm (d50 determined via ultracentrifuge according to J. Coll. Polym. Sci, 267 (1989), 1113). Here, a water-in-oil emulsion is first converted into an oil-in-water emulsion via exposure to shear (phase inversion). The water-in-oil emulsion used is composed of an organic phase of a water-immiscible, organic solvent, in which the polymer has been dissolved, and of an aqueous phase. If the said process is to be carried out successfully it is essential that the specific viscosity of the organic phase is in the range from 1.0 to 20 000 mPa×s (measured at 25° C.), the surface tension between organic and aqueous phase is from 0.01 to 30 mN/m, the particle size of the water emulsified in the organic phase is from 0.2 to 50 μm and the ratio by volume of organic to aqueous phase is in the range from 80:20 to 20:80. The shear must moreover be applied with a specific shear rate of from 1×103 to 1×108 watts per cm3. The list of polymers that can be used in this shear process is very varied, one option inter alia being hydrogenated nitrile rubber. Again, the use of emulsifiers is described as a requirement for this phase-inversion process, and the examples use 4.7 parts by weight for 100 parts by weight of polymer. The organic solvent is removed conventionally, e.g. via distillation, depressurization, reverse osmosis, cyclone devolatilization or spraying through a nozzle. The emphasis in EP-A-0 863 173 is on butyl rubbers and halogenated butyl rubbers. In this case, the specific shear process can produce latices with average rubber particle sizes of from 0.28 to 1.9 μm.
A disadvantage shared by the abovementioned prior-art documents is that the resultant latices comprise relatively large amounts of emulsifiers. The said emulsifiers either derive from the actual NBR emulsion polymerization process or have to be added separately during production of the latex in order to stabilize the oil-in-water emulsion which is an intermediate stage in production of the latex. Without emulsifier it is impossible to achieve a stable oil-in-water emulsion, i.e. the emulsifier is needed to achieve stability of the liquid “oil” phase (rubber in solvent). At the said juncture, the rubber is not yet present in the form of solid. Without emulsifier, the resultant droplets immediately recoalesce, and it is impossible to achieve a fine-particle suspension after a process of concentration by evaporation. The disadvantages of the presence of emulsifier have been described in detail above.
Starting from the prior art, the object of the present invention consisted in providing particularly fine-particle aqueous suspensions which are nevertheless stable and which comprise completely or partially hydrogenated nitrile rubbers with high solids concentration, where the emulsifier content thereof should be minimized. The content of double bonds should moreover be adjustable within a wide range, and a content of less than 5% should especially be achievable. The fine-particle suspension should as far as possible comprise no gel. Another object of the invention was to permit production of these suspensions by way of a process which is simple and does not require major resource in terms of apparatus.